1. Field of the Invention
The invention relates to an image heating apparatus for heating an image formed on a recording material. As such image heating apparatus, mention can be made, for example, of a fixing apparatus which heat-fixes a toner image on a recording material formed by the use of an electrophotographic printing method or an electrostatic recording method.
2. Related Background Art
There has heretofore been devised and put into practical use an apparatus for heating and fixing a toner image in a powder material form formed of heat-fusible resin by image forming process means as a fixed image on a recording material by a fixing apparatus.
As a fixing apparatus for heating and fusing a toner image to thereby fix it on a recording material, there is known one using a fixing roller and a pressure roller, or one using a fixing roller and a pressure belt.
In such a fixing apparatus constituted by a fixing roller and a pressure belt, it has been proposed to make the pressure belt movable toward and away from the fixing roller so as to keep the pressure belt spaced apart from the fixing roller except during sheet supply (see Japanese Patent Application Laid-open No. H11-231701). This is for preventing an inconvenience caused by vapor in paper having its surface coated with resin or the like, i.e., so-called coat paper, breaking through a coat layer and diffusing when the coat paper is subjected to a fixing process. In this apparatus, the controlled temperature of the pressure belt is made lower than the controlled temperature of the fixing roller.
So, in the above-described fixing apparatus, immediately before a recording material dashes into a nip portion, the pressure belt is brought into contact with the fixing roller to thereby decrease an amount of heat given from the pressure belt side to the recording material (unfixed image) (provide a predetermined or greater temperature difference between the fixing roller and the pressure belt), thus preventing the above-noted inconvenience.
In the above-described fixing apparatus, however, there is adopted a construction in which the pressure belt is brought into contact with the fixing roller at uniform timing immediately before the recording material dashes into the nip portion. That is, the time from a point of time at which the pressure belt has been brought into contact with the fixing roller until a point of time at which the recording material dashes into the fixing nip is uniformly determined. Therefore, the following problem has arisen in a case where after the fixing process has been continuously carried out in an image forming job of continuously effecting image formation on a plurality of recording materials, the next image forming job is immediately demanded.
In a case where the fixing apparatus is operated at the start of that next image forming job, if the temperature of the pressure belt at that point of time is low, the pressure belt has sometimes not come to rise to a predetermined temperature before the recording material dashes into the nip portion, and the amount of heat given to the recording material (image) has become deficient, thus causing faulty fixing. On the other hand, if the temperature of the pressure belt at that point of time is high, the temperature of the pressure belt will become nearly equal to the temperature of the fixing roller and the amount of heat given to the recording material (image) will become excessive. As a result, thus has sometimes been caused the occurrence of a faulty image by high temperature offset or the above-described vapor diffusion (in the case of the coat paper), or the faulty fixing that the recording material is not separated from the pressure belt.